1. Field of the Invention
The present invention relates to an image forming apparatus for primarily transferring a toner image formed on an image bearing member onto an intermediate transfer member and, thereafter, secondarily transferring the toner image onto a recording material. More particularly, the invention relates to control for reducing an influence of a resistance change of an intermediate transfer member on an image that is caused when a reverse bias voltage adapted to prevent a detection toner image, such as a color patch or the like, from being transferred onto the intermediate transfer member has been applied.
2. Description of the Related Art
There has been put into practical use an image forming apparatus constructed in such a manner that toner images of separation colors are sequentially primarily transferred and overlaid onto an intermediate transfer member and the overlaid toner images are secondarily transferred from the intermediate transfer member onto a recording material together, thereby forming a full-color image.
In Japanese Patent Application Laid-Open No. 2005-308931, there has been disclosed an image forming apparatus which has a developing device of a developing color rotation switching type and which is constructed in such a manner that toner images of separation colors are sequentially formed by one photosensitive drum (image bearing member) and are sequentially overlaid onto an intermediate transfer member. A secondary transferring apparatus for secondarily transferring the toner images from the intermediate transfer member onto the recording material together can come into contact with or can be separated from the intermediate transfer member. An intermediate transfer member cleaning device of an electrostatic fur brush type is arranged on a downstream side of the secondary transferring apparatus.
In Japanese Patent Application Laid-Open No. 2006-98473, there has been disclosed an image forming apparatus constructed in such a manner that a detection toner image (color patch) for concentration detection formed on a photosensitive drum is optically detected on a surface of the photosensitive drum just after a developing device and a result of the concentration detection is fed back to a developing voltage.
According to the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2006-098473, it is necessary that the detection toner image is promptly removed from an intermediate transfer member after the concentration detection so as not to be overlaid on a toner image of an actual image. One of the methods of removing the detection toner image is a method whereby the detection toner image is primarily transferred onto the intermediate transfer member and circulated and the detection toner image is removed by an intermediate transfer member cleaning device on the downstream side of a secondary transfer position.
However, there is a case where the detection toner image which has primarily been transferred onto the intermediate transfer member cannot be sufficiently removed by the intermediate transfer member cleaning device. For example, in the case where an electrostatic fur brush is used in the intermediate transfer member cleaning device as disclosed in Japanese Patent Application Laid-Open No. 2005-808931 or an intermediate transfer belt is used as an intermediate transfer member, it is difficult to sufficiently remove the detection toner image.
Therefore, there has been proposed such a technique that the detection toner image is allowed to pass through a primary transfer position without being primarily transferred onto the intermediate transfer member and the detection toner image is removed by a drum cleaning device provided for the photosensitive drum. An electric field in the direction opposite to that upon ordinary primary transfer of the toner image is made to act at the primary transfer position, thereby preventing the detection toner image from being primarily transferred onto the intermediate transfer member.
However, it has been confirmed that if the detection toner image is repetitively formed in a same area in the moving direction of the intermediate transfer member and a voltage of the same polarity as a charging polarity of the toner image is repetitively applied to such an area, a transfer fluctuation occurs between such an area and an area before/after such an area. That is, if the normal toner image is primarily transferred in place of the detection toner image into the area where the detection toner image has repetitively been formed and is secondarily transferred onto the recording material, concentration of the image formed on the recording material becomes uneven and image quality is deteriorated. It has been confirmed that each time the voltage of the same polarity as that of the toner image is applied, a difference between a resistance value in a thickness direction of the intermediate transfer member in such an area where such a voltage has been applied and the resistance value in an area where a normal transfer voltage is applied increases gradually, so that a transfer efficiency difference of the toner image occurs for the same primary transfer voltage.